Direct production of graphite fibers



United States Patent 3,449,077 DIRECT PRODUCTION OF GRAPHITE FIBERSDagobert E. Stuetz, Westfield, N.J., assignor to Celanese Corporation,New York, N .Y., a corporation of Delaware No Drawing. Filed Feb. 13,1967, Ser. No. 615,374 Int. Cl. C01b 31/07 US. Cl. 23-2091 12 ClaimsABSTRACT OF THE DISCLOSURE The continuous preparation of uniformly highmodulus graphite fibers from preoxidized polybenzimidazole fibers bycontrolled passage of said-fibers through certain fuel/ oxidant flames.

In mans search for high performance materials, considerable interest hasfocused on graphite fibers. Graphite fibers can be defined as fiberswhich consist essentially of atomic carbon and which have an X-raydifl'raction pattern characteristic of graphite. Carbon fibers, on theother hand, can be defined as fibers in which the bulk of the fiberweight can be ascribed to atomic carbon and which show an amphorousX-ray diffraction pattern. Graphite yarns generally have a much highermodulus and higher tenacity than carbon fibers and in addition areelectrically and thermally conductive.

Industrial high performance materials of the future will most likelyinvolve reinforced composites, and graphite fibers theoretically haveamong the best properties of any fiber, including boron, for highstrength reinforcement. Among these desirable properties are highcorrosion and temperature resistance, low density, high tensile strengthand most important, high modulus. Graphite is one of the very few knownmaterials whose tensile strength increases with temperature. Uses forsuch graphite-reinforced composites include aerospace structuralcomponents, rocket motor casings, deep-submergence vessels and ablativematerials for heat shields on re-entry vehicles.

One of the major factors retarding the large-scale use ofgraphite-reinforced composites is the extreme production costs ofuniform high modulus graphite fibers. Although production of fibrouscarbon by pyrolysis of hydrocarbon gases or by discharge between carbonelectrodes has been reported, these methods are not suitable forindustrial applications requiring good quality control. Graphitizationof organic fiber precursors appears to be the only practical industrialroute to graphite fibers.

Most of the prior art methods for producing graphite fibers involve longprocessing periods, high power requirements and/ or expensive and bulkyheating apparatus such as closed furnaces. The processing and equipmentcosts often dwarf the fiber raw material costs. Often the fiber is ofinferior quality due to damage in one or more steps of the treatment.For example, carbon yarn can be converted to graphite yarn by furnacegraphitization in a high temperature vacuum oven. The hot zone materialis generally a metal such as tungsten or tantalum. Besides beingexpensive and slow, this radiant heat method also may result in thedeposition of foreign materials such as tungsten, tantalum and/ orcarbides of these metals on the fiber during the high temperaturetreatment.

Another prior art approach to graphitizing carbon fiber, calledconductive graphitization, involves passing the fiber over electrifiedrollers. For example, in one such method the carbonized fiber isadvanced over a pair of spaced electrical rollers while supplying anelectric current through the advancing fiber to raise it tographitization temperature. A controlled atmosphere of hydrogen, carbonmonoxide, ammonia or mixtures thereof generally must be provided aroundthe fiber during passage from one roller to the other. Additionally,equipment must be ice is a high incidence of inhomogeneity along thefinished yarn. Many of the fibers have low tensile strength andgenerally there is a wide distribution of individual break values.Apparently the combination of frictional contact of the yarn on therolls during conductive graphitization and the arcing causes marked wearon the yarn such as to produce many flaws.

Another process for producing graphite fibers is disclosed in copendingapplication Ser. No. 614,811 filed Feb. 9, 1967. According to saidprocess suitable carbon fibers are passed through certain fuel/oxidantflames. Carbon fibers utilizable in such a process are characterized by(1) at least and preferably at least of its total weight attributable toatomic carbon, (2) a relatively amorphous X-ray diffraction pattern, and(3) structural integrity for at least two seconds at a fiber temperatureof 1,900" C. Such suitable carbon yarns are in turn generally preparedby the controlled preoxidation of an organic fiber followed by acontrolled pyrolysis. Such preoxidizing and carbonizing procedures aredisclosed, for example, in U .5. Patents Nos. 3,053,775, 3,107,152,3,116,975, and 3,235,323; French Patent NO. 1,430,803; Japanese PatentNo. 12,615/63 and British Patent No. 1,034,542. A wide variety oforganic fiber precursors of diverse chemical nature can be successfullyemployed for this purpose.

Graphite fibers produced in this manner have a relatively higher averagetensile strength and a much narrower distribution of individual breakvalues than is obtainable by conductive graphitization. This improvementis apparently due to two factors-the flame healing of flaws by ionizedcarbon fragments in a luminous flame and the reduced mechanical wear ofthe yarn in the flame is contrasted to the frictional contact of therolls during conductive graphitization.

While the process of the aforementioned copending application itself iscontinuous, rapid, simple, and inexpensive and does not require bulky orexpensive equipment, the preparation of the carbon fiber startingmaterial itself is laborious and expensive. The pyrolysis procedures aregenerally slow and batch-wise. The necessary heating elements are bulkyand expensive.

It is an object of this invention to continuously produce graphitefibers having uniformly good tensile properties without the need for aslow and expensive pyrolysis step.

I now have discovered that graphite fibers having the aforementioneddesirable properties can be prepared directly from polybenzimidazolefibers preoxidized under controlled conditions. According to the methodof this invention these preoxidized polybenzimidazole fibers aredirectly flame graphitized, i.e.. subjected to the flame graphitizationstep described in the aforesaid copending application. 'If preoxidizedfibers other than polybenzimidazoles, such as cellulosics, are directlysubjected to flame graphitization conditions they lost their structuralintegrity, i.e. they burn. Thus, by employing preoxidizedpolybenzimidazole fibers, graphite yarns can be directly, rapidly,easily and cheaply produced.

Polybenzimidazoles are a known class of heterocyclic polymers. They areprepared and described in Patent Nos. 2,895,948 and 3,174,947, forexample. An espesially interesting subclass of polybenzimidazoles forfiber production consists of recurring units of the formula:

wherein R is an aromatic nucleus having each of the two depicted pairsof nitrogen atoms substituted upon adjacent carbon atoms of the saidaromatic nucleus and R is a carbocyclic aromatic or alicyclic ring, analkylene group, or a heterocyclic ring. Examples of such heterocyclicrings include pyridine, pyrazi'ne, furan, quinoline, thiophene andpyran. Preferred R groups are 3,3, 4,4-

bisphenylene and 1,2,4,5-phenyleneand wherein R" is wherein Z is anaromatic nucleus having the two depicted nitrogen atoms substituted onadjacent carbon atoms of said aromatic nucleus. Examplary of suchpolybenzimidazoles is poly-2,5 (6) -benzimidazole.

The most important polybenzimidazole commercially is poly 2,2 n1phenylene 5,5 bibenzimidazole which consists of recurring units of theformula:

This species is commonly referred to as simply PB-I. A preparation ofFBI is described in Example 11 of Patent No. 3,174,947. The fiber can bedry spun from dimethylacetamide, for example, in a manner known to theart.

The preoxidation step is conveniently carried out by heating the yarn inair at about 400500 C., preferably 430500 C., for about 2 to 15 minutesand preferably for 3 to 9 minutes. At the higher temperatures of thisrange a shorter exposure time can be employed. The preoxidation can alsobe effected chemically by the use of reagents such as nitric acid andpotassium dichromate.

Undrawn, drawn and double drawn polybenzimidazole fibers can bepreoxidized and flame graphitized according to the manner of thisinvention.

The nature of the reducing flame is not critical for operability butspecific types of flame result in better tensile properties and/orgreater ease of operation.

A highly preferred flame is that resulting from a acetylene and oxygenmixture. With this flame, the graphitizing step can be conducted in openatmosphere. A further advantage of the acetylene-oxygen flame is that ithas a fairly constant high temperature which is independent, withinlimits, of the fuel/oxygen ratio. A carbon monoxide-oxygen flame alsoprovides good results in an open atmosphere, although this of courserequires safety provision for the operator. Hydrocarbon fuels such aspropane and butane are operable but the process does proceed as smoothlyas with carbon monoxide or acetylene. In the presence of an inertblanketing gas in the processing chamber, comparable stability isachieved with hydrocarbon fuels.

Molecular oxygen can be replaced in the combustion mixture by a gaseousoxidant such as nitrous oxide although generally it is not advantageousto do so since oxygen is so convenient. Fuel/oxidant combinations can beemployed which do not contain a hydrocarbon such as a carbonmonoxide-hydrogen mixture and a hydrogenchlorine mixture.Non-conventional flame sources such as augmented flames (cf. B.Karlovitz, International Science of Technology, June 1962, pp. 3641) andrecombination flames such as the atomic hydrogen torch (cf. I. Langmuir,Industrial & Engineering Chemistry, June 1927, pp. 667-674), plasmatorches and the like can also be employed to provide high temperatures.The temperature should not be so high, however, as to destroy the fiber.

In the context of this specification, temperatures in the flame zonerefer to the temperature of the yarn as measured by an infra-redradiation thermometer and not to theoretical temperature under adiabaticconditions, i.e. without withdrawal of heat by immersing a body into theflame. The yarn temperature in the flame is generally significantlylower than the theoretical flame temperature. For example, thetheoretical flame temperature of an oxyacetylene flame is about 3,100 C.An upper limit of about 2,500 C. for the yarn temperature is generallysufficient and safe.

The fuel to oxidant ratio generally is a significant parameter. Thegraphitizing tereatment is best carried out in a luminous flame obtainedby keeping the amount of oxygen in the fuel mixture below thestoichiometric amount which is required to burn the fuel completely byoxidation if the oxidant-fuel ratio is too high. The luminosity of theflame is believed to be caused by ionized carbon fragments in the flamedue to incomplete combustion. More pyrolytic carbon will be deposited athigher oxygen fuel ratios than at lower ratios. For certain applicationsa deposit of pyrolytic graphite is desirable since it increases the hightemperature stability of the yarn; for others it is undesirable, e.g.,where good adhesion to a matrix is desired. Hence, the flexibility ofprocessing conditions allows the production of a variety of graphiteyarn types. A surface protective layer of this kind can be formed in aseparate step in which the yarn is heated to high temperatures in acontrolled environment containing hydrocarbon vapors.

The volume flow of the fuel and oxidant through the burner should be ashigh as possible, consistant with good flame stability, in order tomaximize the moduli of the fibers.

The carbon yarn must be passed through the flame at a fast enough rateto avoid breaking. As the flame temperature is increased, the minimumrate at which breakage is avoided also increases. This minimum speed canbe determined for any given combination of yarn and flame. The longerthe residence time, the greater the extent of graphitization. Optimumconditions are reached at the point where loss of fiber mass by burn-offis lowest and conversion of the remainder of graphite is highest. Thetwo effects can be balanced favorably by adjusting resi deuce time andyarn temperature. Subject to the nature of the flame and other factors,residence times of 2 to 24 seconds, and preferably 6 to 17 seconds aregenerally suitable. An exemplary set of optimum conditions is a yarntemperature of 2,300 C. with a residence time of about 15 seconds.

Tension during flame treatment is important in achieving optimum yarnproperties as it prevents the tendency of the yarn to shrink. Shrinkageusually leads to relaxation of ordered structures and, thereby, causeslowering of physical properties. Preservation of orientation and/orincrease of orientation, depending on the magnitude of tension applied,increases both Youngs modulus and tensile strength. The tension appliedshould be at least sufiicient to avoid visible sagging. Beyond theoptimum tension the fiber may be damaged by still higher tensions. Thetension can be adjusted to a level where the denier size of the yarn ispreserved or even slightly reduced by drawing. Tensions should be about25 to 600 grams per 1,800 denier of precursor yarn and preferablybetween 150 and 300 grams. The amount of desirable tension depends onwhether the precursor yarn is undrawn, single-drawn or double-drawn andincreases generally in this direction.

Treatment of the polybenzimidazole yarn priorto flame passage with anaqueous boric acid solution (e.g. 20%) may result in a slight increasein the modulus of the graphite yarn as compared to non-treated yarns.Other flameproofing materials which can be optionally employed includesilicone oil (DOW 700), antimony salts and the common bromine-andchlorine-containing flameproofiing compounds.

The necessary apparatus for flame graphitization is simple and should beso arranged that the yarn is exposed to a minimum of frictionalcontacts. A convenient setup is to feed the preoxidizedpolybenzimidazole yarn from a rotating-reciprocating bobbin through theflame to an identically functioning take-up mechanism. Starting at thecorrect reciprocating position on the bobbin, the yarn is unwoundwithout traverse movement and analogously rewoun d at the take-up side.Hence, random yarn motions I are minor. Further positioning of the yarnin the flame can be accomplished with minimal action by twocylindrically shaped guides located before and after the bumers. Feedand take-up bobbins can be driven by, for example, solid-statecontrolled D.C. motors with r.p.m. generator/indicators. When an inertatmosphere is desired, a cruciform glass vessel fitted with coolingplates and passage opening for the yarn can be employed. Before enteringthe burner module, the yarn is put under constant tension as, forexample, by passing it over a rubber-capped electro-magnetic clutch anda skewed roll. The latter separates individual yarn loops around theclutch and prevents abrasion by yarn to yarn contact.

The geometry of the burner is also a factor in maximizing theeflectiveness of the flame graphitization of this invention. Twoimpinging flames originating from two standard conical tipssignificantly raise the temperature of the yarn passing therethrough. Aparticularly preferred embodiment of the latter method is theimpingement of the two flames on the tips of their inner cones at anangle of forty-fiive degrees. However the addition of more than twoorifices does not have a beneficial eflect. A series of burners such asto form a continuous flame zone of increased lateral dimension maypermit higher processing speeds. Since residence time in the flame is amajor parameter, processing speed is significantly related to the lengthof the flame zone. Surrounding the conical tip with a cylindrical orglobular reflector, constructed from" polished stainless steel sheets,for example, also significantly raises the yarn temperature.

The following tables illustrate the tensile properties of graphitefibers obtainable by the method of this invention as a function of (1)the type of PBI yarn employed, including type of preoxidizing anddrawing, (2) the residence time in the flame, (3) the tension on theyarn during passage through the :flame, (4) the absolute and relativeflow rates of the acetylene and oxygen and (5) the type of burneremployed.

The yarn types designated in the tables are coded on the basis of thenature of their precursor PBI yarn and the preoxidization conditions asfollows:

APrecursor 1,800 denier/450 filament, preoxidized at 485 C. for 5.0minutes.

A Same as A, but impregnated with a 5% aqueous solution of boric acidand dried.

A Same as A, but coated with silicon oil (Dow Corning A Same as A, butpreoxidized under gram ten- A -Same as A, but preoxidized under 200 gramten- BPrecursor 1,800 denier/450 filament, preoxidized at 485 C. for 4.5minutes.

B --Same as B, but preoxidized under 450 gram tension, at a profile of435-485 C. (3-zone).

B Same as B but under 650 gram tension.

C-Precursor 1,800 denier/450 filament, preoxidized at 485 C. for 3.75minutes.

BPrecursor 2,000 denier/600 filament, preoxidized at 485 C. for 3.75minutes.

BPrecursor undrawn, 3,800 denier/450 filament, preoxidized at 485 C. for5.0 minutes.

F-Precursor double drawn, 1,530 denier/450 filament,

preoxidized at 485 C. for 5.0 minutes.

HPrecursor double drawn, 1,530 denier/450 filament,

preoxidized at 485 C. for 6.0 minutes.

IPrecursor double drawn, 1,700 denier/500 filament,

preoxidized at 485 C. for 6.0 minutes.

K-Precursor double drawn, 1,700 denier/500 filament,

preoxidized at 485 C. for 5.0 minutes.

The FBI precursor itself has tensile properties as follows:

TABLE I Tensile strength Initial modulus Yarn type (10 p.s.i.) (10p.s.i.)

Undrawn 33 0. 4-0. 8 Single Drawn- 87 1. 9 Double Drawn 2. 3

In the above code, the FBI precursor yarn is single drawn unlessotherwise noted.

In the runs in Table II, the standard flame employed resulted from acombination of acetylene flowing at a rate of 1,150 mL/minute and oxygenflowing at the rate of 750 ml./minute. The burner had a standard conicaltip, 0.050 inch in internal diameter.

Table III gives further data on the embodiment of this invention whereintwo standard conical tips of the types used in Example I are employed informing the flame zone. This listed flow rate represents the total flowrate from both burners.

TABLE III Time in Acetylene/ Tensile Initial Tension Flame 0; ratiostrength modulus (grams) (sec) (ml/min.) (10 p.s.i.) (10 p.s.i.)

TABLE IV Tensile Initial Tension Time in strength modulus Yarn type(grams) flame (see) (10 psi.) (10 psi.)

B 200 6 155 25. 300 6 176 28. 0 400 6 208 28. 6 13m 200 6 176 29. 0 30012 156 30. 0 E 200 3 142 17. 6 F 200 3 176 19. 0 300 3 120 18. 4 H 300 3144 19. 0 450 3 110 21. 0 400 12 128 23. 2 600 12 146 24. 6 I 500 3 17022. 0 K 400 3 150 22. 0

Tables II through IV illustrate the improved tensile propertiesachievable by the method of this invention.

Although the above discussion has focused on the use of continuousyarns, staple yarns may also be used. They will generally givecorrespondingly lower tensile properties than will continuous yarns.

Specific embodiments of the invention can be optimized for desiredproperties and/or processing parameters by simple experimentation andnumerous relationships with other variables will become apparent. Forexample, other factors constant, the finer the denier the higher themodulus.

By the method of this invention one can form even substantial packagesof graphite yarn, i.e. one containing more than 10 grams, in which allportions of the yarn have (1) an X-ray diffraction patterncharacteristic of graphite, and (2) an average deviation from both theYoungs modulus value and the average tenacity value of less than -5%.Ten grams of the graphite yarn of this invention is of the order ofmagnitude of 100 feet in length. Such a package of uniform graphite yarnis particularlysuited in those applications where property uniformityand reliability are necessary.

Numerous other variants of the above process and product will beapparent to one skilled in the art within the spirit of this invention.

What is claimed is:

1. A rapid process for the production of uniform graphite fiberscomprising the steps of preoxidizing a polybenzimidazole fiber andpassing said p-reoxidized fiber through a reducing flame imparting tothe yarn a minimum temperature of at least 1,900 C. 'at a speedsufiicient, to avoid breaking and under a tension at least sufiicient toavoid visible sagging.

2. A process according to claim 1 wherein said flame is generated by afuel'oxidant mixture.

3. A process according to claim 2 wherein said oxidant is oxygen.

4. A process according to claim 3 wherein said fuel is acetylene.

5. A process according to claim 3 wherein said fuel is propane.

6. A process according to claim 1 wherein said polybenzimidazole ispoly-2,2-m-phenylene-5,5'-bibenzimida zole.

7. A process according to claim 1 wherein said preoxidizing step isconducted by heating in air at about 400550 C. for about 2 to 15minutes.

8. A process according to claim 1 wherein said preoxidizing step isconducted by heating in air at 430500 C. for 3 to 9 minutes.

9. A process according to claim 2 wherein the residence time of thefiber in the flame is from 2 to 24 seconds.

10. A process according to claim 3 wherein the ratio of oxygen and fuelis such that the amount of oxygen is below the stoichiometric amountrequired to completely oxidize the fuel.

11. A process according to claim 5 wherein an inert atmosphere isprovided around said flame.

12. A process according to claim 8 wherein the fuel is acetylene and theoxidant is oxygen and the fiber is passed through the luminous portionof the flame for a residence time of 6 to 17 seconds.

References Cited UNITED STATES PATENTS 3,011,981 12/1961 SOltes 252-5023,107,152 10/1963 Ford et a1. 23209.2 3,174,947 3/1965 Marvel et a126047 3,285,696 11/1966 Tsunoda 23209.1 3,304,148 2/1967 Gallagher23209.2 X 3,313,597 4/1967 Cranch et al. 232093 EDWARD J. MEROS, PrimaryExaminer.

U.S. Cl. X.R.

